Juan de Fuca Ridge - Regional Geophysics

Regional Geophysics of the Juan de Fuca Ridge

Maurice A. Tivey - Woods Hole Oceanographic Institution

Abstract of talk given at NSF RIDGE Juan de Fuca Symposium, Seattle, 1999

One of the most enduring geophysical images of the Juan de Fuca Ridge (JDFR) is the Raff and Mason (1961) magnetic anomaly map collected in the late 1950's. Fred Vine and colleagues used this map to demonstrate a fundamental observation of plate tectonic theory that magnetic anomaly stripes were a record of Earth's magnetic field polarity reversal history preserved by ocean crust as it formed and moved away from a midocean ridge. The discovery of a midocean ridge (MOR) spreading center off the northwest coast of North America was first postulated in the late 1950's with the discovery of an undersea ridge and valley terrain. McManus and colleagues [1964] produced the first bathymetric map of the JDFR defining its length (500 km), and location, north of Cape Mendocino. Subsequent dredging studies revealed the relative young age of the volcanic rocks along the ridge confirming this area as a MOR system. A subsequent systematic regional geophysical study was carried out by the National Ocean Survey [Elvers et al., 1974] utilizing 12 kHz bathymetry, magnetics, gravity and single channel seismics. In the early 1980's it was recognized that the JDFR is composed of more than one ridge segment with the discovery of the offset in the spreading center at Axial seamount [Delaney et al., 1981]. The 1980's will be remembered, however, for the revolution in seafloor mapping with the introduction of swath bathymetry systems such as Sea Beam. These multibeam echo sounders provided the first clear documentation of the segmented nature of the JDFR spreading center. The JDFR can be divided into 7 main ridge segments from south to north : Cleft, Vance, Axial (including North rift and South rift), CoAxial, Cobb, Endeavour, and West Valley. The longest of these segments is Cobb at 150 km, while the shortest is CoAxial at 75 km. Each of these rift segments show quite different morphologies and volcanic and tectonic settings that span the evolutionary history of seafloor spreading at the JDFR. One of the unforeseen uses of swath bathymetry mapping was only realized years later when repeat mapping was utilized to identify and date volcanic eruptive units on the seafloor. It is clear that repeat swath mapping is an absolute requirement for benchmark studies and higher resolution systems (e.g. EM300) provide a unique opportunity to establish the appropriate scale basemap with which to compare all future eruption and deformation events. The 1980's also saw the introduction of digital sidescan sonar systems to the JDFR from the broad-scale GLORIA system to surface towed SeaMARC II and deep-towed SeaMARC I and Scripp's Deeptow.

Regional gravity work on the JDF reveals that Axial seamount is underlain by a Mantle Bouguer gravity Anomaly (MBA) low which dissipates southwards to the Blanco Fracture zone [Hooft and Detrick, 1995]. South of the Blanco, the Gorda ridge has virtually no MBA anomaly. Gorda ridge spreads at the same rate as the JDFR and is extremely seismically active while the JDFR is aseismic. The MBA gravity contrast suggests that there is thicker crust and hotter mantle at Axial with a thinner and weaker lithosphere while Gorda Ridge has a strong, thick lithosphere, hence the seismicity, but a cold mantle and thin crust. A thermal anomaly is postulated deep beneath the Axial seamount edifice, compatible with a detailed gravity analysis of Axial seamount which finds evidence of a low density region within the edifice, possibly a magma chamber [Hildebrand et al., 1990].

From a regional seismic crustal structure viewpoint, only three ridge segments have had detailed studies. The Cleft segment was imaged by a 3 line multichannel seismic (MCS) survey in 1981 [Morton et al., 1987]. Two profiles across the rise axis and one along the axis found an axial magma chamber reflector, 1-2 km wide and 2.3 to 2.5 km deep. The data quality was not good enough to discern polarity but it was suggested that this reflector could be the top of a partially crystallized magma chamber. A later seismic refraction survey imaged the shallow structure at the north end of Cleft segment and found thin extrusive crust (~350 m thick) with significant seismic anisotropy, with the slow direction perpendicular to the axis, presumably due to fractures oriented parallel to the rise axis [McDonald et al., 1994]. Endeavour Ridge was imaged by MCS in 1985 and a narrow (1 km) axial reflector was found at about 2.5 km depth beneath the ridge axis [Rohr et al., 1988]. Again, the polarity of the reflector could not be determined and no obvious velocity anomalies were observed beneath the reflector. A shallow intermittent reflector at 600-1000 m depth that followed the seafloor was also imaged and presumably indicates the base of layer 2A. A moho reflection west of the axis was observed at 6.6 km depth. Detailed travel-time analysis of seismic refraction data (SEISRIDGE 85) at Endeavour revealed a three-layer crustal sequence: an upper layer 2A crust 250-600 m thick, an intermediate layer 800 m thick and a lower layer of dike-like velocities (5.8 km/s) [White and Clowes, 1990]. A low velocity zone beneath and slightly west of the axis was interpreted as a zone of intense fracturing. No evidence for a large magma chamber was found although velocities indicate slightly elevated temperatures (150-250 °C) at depth beneath the axis. CoAxial ridge was the third area studied in detail following the 1993 eruption. A refraction seismic survey revealed the velocity structure down to 2 km depth and found that the central part of the segment had a relatively symmetrical and systematic velocity variation with low velocity on axis, higher velocities off-axis and lower velocities beneath the ridge flanks. This pattern dissipated northwards towards the eruption site which was suggested to indicate a transition from magmatic to more amagmatic extension northwards [Sohn et al, 1997].

The JDFR was the location of some of the earliest heat flow measurements and from the beginning it was noticed that heat flow measurements made near the midocean ridge crest were much lower than those made off axis in older crust. It was Lister [1971] who suggested that hydrothermal circulation within the volcanic basement reduced the heat flow at midocean ridges so that convection rather than conduction dominated the heat transfer process. It has taken about 20 years for technology to enable high-resolution heat flow measurements to be made but now recent data from the flanks of the JDRF demonstrate categorically that the upper volcanic basement is extremely porous and acts as a conduit for the rapid movement of fluids. The Flank Flux study initiated in 1988 on the flanks of the Endeavour ridge completed a closely spaced set of heat flow measurements and revealed increasing heat flow with thickening sediment cover followed by an inverse relationship between sediment thickness and heat flow [Davis et al., 1989]. Thicker sediments have low heat flow and while thin sediments over basement highs have high heat flow. This observation implies an isothermal basement which further implies regional scale permeability. This implication was confirmed with a series of 10 deep drill holes through the sediment column which documents a slow increase in basement temperature as sediment cover thickens followed by an isothermal basement at about 68 °C. The drilling also found that the basement fluids were of a young age 10 kyrs further suggesting the rapid circulation of seawater through the upper basement. The Flank Flux drill holes were also sealed by CORKS to allow study of the in situ pressure regimes. These studies reveal the potentially important influence of tidal forcing on fluids in the basement as well as local topographic gradients. The heat flux through bare rock, non-sedimented terrain remains an unknown quantity at present due to difficulties in making heat flux measurements. Recent progress has been made using thermal blanket technology [Johnson and Hutnak, 1997] and these studies may help to understand circulation within young basement, recognizing that fissures and faults will likely dominate permeability and heat flow patterns.

Finally, recent experiments have aimed at trying to determine the ground and crustal movements related to seafloor spreading and volcanic eruptions. The frequency of spreading events remains unclear and the partition from the plate motion scale down to a diking event scale is still unknown. Several groups have placed devices to measure short baselines across the rise axis, but a consistent pattern has yet to be seen but this will likely be a long term project [Chadwick et al., 1997; Chadwell et al., 1999]. Other tiltmeter and ground deformation experiments on Axial seamount volcano have also detected the inflation and deflation of the presumed magma chamber [Fox, 1995]. These monitoring devices will likely play an increasingly important role in understanding the dynamics of undersea volcanic system just as they have on land. Our knowledge of volcanism in the deep ocean will ultimately require the scientific community to establish continuous in-situ sensors over a long time period to make progress in this area.

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